This invention relates to a magnetic resonance imaging system (hereinafter referred to as an MRI system), in which a magnetic resonance (MR) phenomenon in a specific atomic nucleus is excited in an object to detect its MR data and to obtain an MR image with respect to a selected portion of the object and, more particularly, to an MRI system which makes it possible to obtain a suitable magnetic field.
In a well-known MRI system, an object is disposed in a static magnetic field, and a high frequency magnetic field orthogonal to the static magnetic field is applied to the object to produce a MR phenomenon while X-, Y- and Z-axis gradient fields are suitably superposed on the static magnetic field to detect a MR signal from the object so as to obtain an image based on the MR data.
In order to obtain a high quality image with this MRI system, it is desired that the X-, Y- and Z-gradient fields have a rectangular waveform as shown by the solid line in FIG. 1A. To obtain such a field waveform, pulse current is supplied to each gradient coil.
However, eddy current based on the gradient field is produced in conductive members adjacent to the gradient field coil, e.g., in frame, coils and shield member for the superconduction magnet. Due to the influence of the field that is produced by the eddy current, the waveform of the gradient field actually applied to the object is distorted as shown by the broken line in FIG. 1A. Therefore, the intensity of the applied gradient magnetic fields is not constant but varies with time. An image having satisfactory quality, therefore, can not be obtained by imaging using the gradient magnetic fields noted above. To compensate for the distortion of the waveform of the gradient magnetic field, a current, including an overshoot as shown by the solid line in FIG. 1B is supplied to generate the gradient magnetic field to make up for a change in current due to the waveform distortion problem. By so doing, the waveform can be corrected to a regular rectangular waveform as shown by the broken line in FIG. 1B.
However, another problem results when the conductive members and a gradient magnetic field coil, e.g., a Y-axis gradient magnetic field coil for generating a Y-axis gradient magnetic field (or an X-axis gradient magnetic field coil for generating an X-axis gradient magnetic field or a Z-axis gradient magnetic field coil for generating a Z-axis gradient magnetic field, although the Y-axis gradient magnetic field is considered for illustration in this example) is arranged asymmetrically with respect to the Y-axis direction, that is, when the distances D1 and D2 of upper and lower coils 2A and 2B of Y-axis gradient field coil 2 noted above, consisting of upper and lower coils 2A and 2B, from conductive member 1 are related as D1&gt;D2.
The problem is that the center of the Y-axis gradient magnetic field and the center of a field that is produced by eddy current in the conductive member do not coincide but are rather deviate from each other.
Such positional deviation of the field centers will now be described specifically with reference to FIG. 3. In FIG. 3, the ordinate is the magnetic field intensity, and the abscissa is position (i.e., displacement) in the Y-axis direction. In the graph, line La represents a field set up by upper coil 2A, line Lb a field set up by lower coil 2B, line Lc a resultant field obtained from the fields of lines La and Lb, line Ld a field set up by eddy current due to the field of upper coil 2A, line Le a field set up by eddy current due to the field of lower coil 2B, and line Lf a resultant field obtained from the fields of lines Ld and Le. As is obvious from the graph, the center of the gradient magnetic field represented by line Lc, which is the resultant of the fields of lines La and Lb, passes through the origin O. The center of the eddy current field represented by line Lf, which is the resultant of the fields of lines Lc and Ld, does not pass through the origin O but, instead deviates by ye therefrom.
The center of the two magnetic fields do not coincide. The influence of the field to the eddy current varies with position. For this reason, a regular rectangular waveform as shown by the broken line in FIG. 1B can not be obtained even if a current including an overshoot is supplied as discussed above.
In an ordinary MRI system, the Y-axis gradient magnetic field equipment specifically has a structure as shown in FIG. 4, consisting of four saddle coil segments 2A to 2D, two of them disposed on each of the upper and lower sides of the space where the object is disposed, i.e., imaging area 3. A gradient magnetic field generation current supplied from a gradient magnetic field signal source 4 through amplifier 5 is supplied to the four coil segments 2A to 2D which are connected in series, as shown in FIG. 5.
As is shown, in the conventional MRI system, if the gradient magnetic field coil and a nearby conductive member are in an asymmetrical positional relation to each other, a gradient magnetic field having a desired rectangular waveform can not be obtained even by supplying a current including an overshoot to the gradient magnetic field.
Further, in a conventional MRI system the gradient magnetic field set up by a gradient magnetic field coil fails to have a desired rectangular waveform not only due to eddy current in a conductive member disposed in the neighborhood of the gradient magnetic field coil but also if there is a magnetic field source providing a magnetic field which is asymmetric with respect to the center of the coil.